implicit int and implicit declaration of functions with gcc compiler

Question

I read in the c99 Standard:

-remove implicit function declaration,

-remove implicit int.

But when I try to compile this code with gcc compiler in c99 mode using -pedantic

main(void){
    f(3);
    return 0;
}


int f(int a){
    ....
}

I expect 2 errors, but I just receive 2 warnings:

-warning: return type defaults to ‘int’

-warning: implicit declaration of function ‘f’.

Shouldn't them be errors in c99?

http://gcc.gnu.org/c99status.html In both situations there's written "done".

Thanks.

-pedantic just means be pedantic in telling the user. If you want the compiler to stop compilation on encountering such things, use -pedantic-errors. However, -Werror may be even better (in conjunction with -Wall -Wextra), since even warnings not required by the standard are undesirable. — Daniel Fischer
– Daniel Fischer, Commented Jun 23, 2012 at 17:49

Keith Thompson · Accepted Answer · 2012-09-19 22:25:08Z

7

The C standard requires a diagnostic for any translation unit containing a violation of a syntax rule or constraint. It does not require such diagnostics to be fatal; the compiler is free to continue processing the source file. The behavior of the resulting executable, if any, is undefined. The standard makes no distinction between warnings and fatal errors.

(The only thing that requires a compiler to reject a source file is the #error directive.)

Conclusion: when compiling C, take warnings very seriously.

edited Sep 19, 2012 at 22:25

answered Jun 23, 2012 at 16:45

Keith Thompson

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Oliver Charlesworth · Accepted Answer · 2012-06-23 16:29:51Z

4

I don't believe the compiler is required to produce a fatal error. Use -Werror if you're concerned...

answered Jun 23, 2012 at 16:29

Oliver Charlesworth

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Jerry Coffin · Accepted Answer · 2012-06-23 16:46:19Z

4

Two points: first, it may (usually does) take a specific set of flags to get a compiler to conform with the standard.

Second, all that's required by the standard is that the implementation issue a "diagnostic" in the case of an error -- but it's up to the implementation to define what is or isn't a diagnostic. It's free to say a "warning" is a diagnostic if it wants to. When a diagnostic is issued, it may quit compiling, or it can compile the code anyway.

Bottom line: what it's doing is probably enough to conform, for whatever that's worth.

answered Jun 23, 2012 at 16:46

Jerry Coffin

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